Recovery of alkali metal salts of benzene-carboxylic acids



y 5, 1965 w. SCHENK ETAL 3,259,651

RECOVERY OF ALKALI METAL SALTS 0F BENZENE-CARBOXYLIC ACIDS Filed Jan.'7, 1965 INVENTORS WALTER SCHENK ALBRECHT WALLIS LUDWlG VOGEL Mal;

ATT'YS Unitcd States Patent s (Ilaims. (or. 26t]525) This application isa continuation-in-part of our copending application Serial No. 211,113,filed July 19, 1962, now abandoned.

The invention disclosed in the present application relates to a processfor the recovery of dry neutral alkali metal salts of benzene-carboxylicacids other than terephthalic acid, suitable for the production ofterephthalic acid by thermal rearrangement.

In the thermal rearrangement of neutral alkali metal salts ofbenzenecarboxylic acids other than terephthalic acid to neutral alkalimetal terephthalates it is advantageous to combine the preparation ofthe initial salts with the liberation of terephthalic acid from theneutral alkali metal terephthalates. This may be carried out for examplein accordance with US. patent specification No. 2,841,615 by reacting,in a first precipitation step, the neutral alkali metal terephthalatewith benzoic acid or with an acid alkali metal salt of abenzenedicarboxylic acid other than terephthalic acid, e.g., withmonopotassium phthalate. Thus, acid alkali metal terephthalate, on theone hand and a alkali metal benzoate or the neutral alkali metal salt ofthe benzenedicarboxylic acid in question, on the other, are obtained inthe form of an aqueous solution. In a second precipitation step,terephthalic acid is set free from the acid alkali metal terephthalateby means of benzoic acid, a benzenedicarboxylic acid other thanterephthalic acid, or the anhydride of such a benzenedicarboxylic acid.Any acid alkali metal salt of the benzenedicarboxylic acid in questionwhich may have been formed is then employed in the first precipitationstep. The above-mentioned aqueous solution of alkali metal benzoate orof the neutral alkali metal salt of the benezenedicarboxylic acid otherthan terephthalic acid is evaporated. In this way, dry initial salt forthe conversion is obtained. 7

It is extremely important that the initial salt be free from any acidsubstances, such as benzoic acid or acid potassium phthalate; otherwisethe yield of terephthalic acid would be decreased considerably. The toaqueous solutions of neutral alkali metal phthalates which are obtainedby the process according to US. patent specification No. 2,930,813 havea pH of about 5.6 and cannot therefore be immediately evaporated to givea suitable initial salt. True, if the solution is adjusted to equivalentpoint by adding caustic alkali solution, preferably caustic potashsolution, a suitable initial salt is obtained; but it is difiicult incommercial operation always to maintain the equivalence point. If toomuch caustic alkali solution is added so that the solution contains freealkali metal hydroxide, there results a marked decrease in the yield ofterephthalic acid, just as in the presence of acid substances. Thisrisk, involved in the use of caustic alkali solution, may be avoided byadjusting the acid solutions of the initial salt to the equivalencepoint with potassium carbonate, because an excess of free potassiumcarbonate in the initial salt does not impair the yield of terephthalicacid. However, in view of the fact that alkali metal hydroxides are muchcheaper than potassium carbonate, it is desirable that potassiumhydroxide should be used for neutralizing acid constituents in theinitial salt solutions.

The recovery of initial salts from their solutions pre-,

sents a further difliculty. If after the addition of alkali hydroxide orcarbonate the solution is first concentrated, then dried in an atomizingdryer by means of burnt gases and the dry salt separated in a cyclone,it is economically advantageous to recover salt dust from the efliuentgas and to recycle the salt dust into the reaction. When, however, theusual cloth filters or electrostatic filters are employed, considerabledifliculty is encountered, since dipotassium phthalate, for example, isextremely hygroscopic. The salt absorbs moisture from the air and thisleads to the blockage of the filter and hence to frequent stoppages inthe operation. Such interference can, on the other hand, also be broughtabout by various types of included catalyst.

It is an object of this invention to provide a process by means of whichdry, neutral alkali metal salts of benzenecarboxylic acids other thanterephthalic acid are obtained which are suitable for thermal conversioninto the salt of neutral alkali metal salts of terephthalic acid. It isanother object of this invention to provide a process in which an alkalimetal hydroxide is used for neutralizing acid constituents in aqueoussolutions of the said neutral alkali metal salts, but in which thepresence of free alkali metal hydroxide in the said dry neutral alkalimetal salts is safely prevented. It is a further object of thisinvention to provide a process in which the said neutral alkali metalsalts are recovered from their solutions practically completely, i.e.,without loss due to entrainment by the gas stream used for drying. It isyet another object of this invention to provide a process in which theabove-mentioned neutral alkali metal salts are obtained continuouslyfrom their aqueous solutions. It is a still further object of thisinvention to provide a process in which the measures serving to preventthe presence of free alkali metal hydroxide in the said dry neutralalkali metal salts and to prevent losses of these neutral alkali metalsalts due to drying by a gas stream, are combined in such a way that anadvantageous over-all efiect is achieved. Further objects and advantageswill be apparent from the following description in conjunction with theaccompanying drawing.

In accordance with this invention these objects are achieved bycontinuously adding to an aqueous solution of alkali metal salts ofbenzenecarboxylic acids other than terephthalic acid, the said solutionhaving a pH below the equivalence point of said alkali metal salts, suchquantities of alkali metal hydroxide that the pH value of the solutionlies 0.05 to 0.9 unit above the point of equivalence, and continuouslydividing the solution into two portions, one of which is evaporated toyield the dry neutral salt of the said benzenecarboxylic acid, and theother is brought into intimate contact with carbon dioxide or a gascontaining carbon dioxide and then returned to the aqueous solution ofthe said alkali metal salts of benbenecarboxylic acids other thanterephthalic acid, before the said first portion is withdrawn, carebeing taken to ensure that such an amount of alkali metal hydrogencarbonate is formed in the second portion of the solution by contactwith carbon dioxide or a gas containing carbon dioxide as to neutralizethe whole excess of alkali metal hydroxide, which brings the pH of thesolutions of alkali metal salts of benzenecarboxylic acids other thanterephthalic acids to 0.05 to 0.9 unit above the equivalence point.

The process according to this invention, then, consists in a cycle fromwhich neutralized salt solution is continuously withdrawn and into whichthere is fed con tinuously a corresponding amount of salt solutionhaving a pH below the equivalence point and alkali metal hydroxide. Thesalt solution, which is being recycled, is treated with carbon dioxideso that alkali metal bicarbonate is formed which neutralizes the excessof free alkali metal hydroxide and thus ensures that the neutralizedsalt solution withdrawn does not contain any free alkali metalhydroxide.

Preferred benzenecarboxylic acids whose alkali metal salts may beobtained by the process according to this invention are benzoic acid,phthalic acid and isophthalic acid. The aqueous solutions of the alkalimetal salts of these acids may be obtained by two-step precipitation asdescribed above, e.g., according to U.S. patent specifications No.2,841,615 or 2,930,813. The solutions usually contain 25 to 45% byweight of the alkali metal salt in question and have a pH which is belowthe equivalence point and usually ranges between 4.0 and 7.0. The alkalimetal salts are preferably potassium and sodium salts; the proportion ofpotassium in the total of the said cations should be 60 to 100% so as toachieve high yields.

Among the alkali metal hydroxides to be added sodium hydroxide andparticularly potassium hydroxide are preferred. The use of sodiumhydroxide is particularly expedient if the sodium content of the saltsolution is low. If the proportion of sodium in the total of the alkalimetal cations is more than 40%, the use of potassium hydroxide only isto be recommended. The alkali metal hydroxide may be added in solid formor as an aqueous solution, advantageously as a 10 to 50 wt.percentsolution. The amount of alkali metal hydroxide may be regulatedby means of a pH controller. The temperature of the alkali metal saltsolutions is not critical; usually it ranges between 20 and 90 C.

It is an essential feature of this invention that the pH of the alkalimetal salt solution is brought to 0.05 to 0.9 unit above the equivalencepoint by addition of alkali metal hydroxide. Thus, in the case ofdipotassium phthalate solutions the pH is adjusted to from 7.95 to 8.8,in the case of potassium benzoate solutions to from 7.35 to 8.2, and inthe case of dipotassium isophthalate solutions to from 7.05 to 7.9. Theupper limit indicated, viz. 0.9 unit above the equivalence point, isdetermined by economic rather than technical considerations. If morecaustic alkali solution is added, an unnecessarily large amount ofalkali metal hydrogen carbonate must be produced by treating the saidsecond portion of the salt solution with carbon dioxide, vin order toneutralize the excess of alkali metal hydroxide. If there are alkalimetal salts of more than one benzenecarboxylic acid other thanterephthalic acid in the solution, the latter is regarded as a solutionof that alkali metal salt whose equivalence point is furthest in thealkaline range.

A first portion of the solution-as a rule 1 to 70%, preferably 5 to25%to which alkali metal hydroxide has been added is continuouslyevaporated in conventional manner to obtain the dry salt, if desired inseveral stages with direct or indirect heating. It is best, at least inthe last stage, to use spray dryers in which flue gas is employed. Thisusually has a temperature between 150 and 400 C. when entering thedryer. In general, 70 to 9 cubic meters (S.T.P.) of hue gas is requiredper kg. of water to be evaporated, according to the temperature of theflue gas. Conventional catalysts for the conversion to be carried out ata later stage may be added to the said first portion prior toevaporation.

The second portion of the solution to which alkali metal hydroxide hasbeen added is recycled continuously. Before that, however, it is broughtinto intimate contact with carbon dioxide or a gas containing carbondioxide. This may be done in conventional manner; the best method is tospray the solution at the upper end of the contact zone into acountercurrent stream of carbon dioxide or a gas containing carbondioxide. The urpose of the treatment with carbon dioxide is to producesuch an amount of alkali metal hydrogen carbonate in the solution (or todissolve such an amount of carbon dioxide in it) that the excess ofalkali metal hydroxide, which has been added to the solutions of thealkali metal salts of the benzenecarboxylic acid in question, isneutralized while the portion treated with carbon dioxide is recycled.The term alkali metal hydrogen carbonate as used herein is meant toinclude the alkali metal carbonate which is formed from the alkali metalhydrogen carbonate proper in amounts varying with the temperature. ThepH of the initial solution, viz. 0.05 to 0.9 unit above the equi valencepoint, is therefore determined by the content of alkali metal carbonateand possibly of alkali metal hydrogen carbonate, but not by alkali metalhydroxide. The salt obtained by evaporation of the said first portion ofthe solution, then, does not contain any free alkali and is thereforemost suitable for the preparation of terephthalic acid.

From the description of the effect of the carbon dioxide treatment itwill be apparent that no limiting data can be given on the amount ofcarbon dioxide or the gas containing carbon dioxide. One factor on whichthe amount depends is the carbon dioxide content of the gas. If line gasis used, this content is from 1 to 15% by volume. Obviously, othergases, especially gases having a larger content of carbon dioxide, andpure carbon dioxide are also suitable. The amount further depends on theintensity of the contact between the gaseous and liquid phases. Thefiner the distribution of the solution sprayed into the gas stream andthe longer the period of contact, the smaller the required amount of gashaving a given carbon dioxide content. The amount of carbon dioxide orgas containing carbon dioxide required for treating the second portionof solution finally depends on the amount of alkali metal hydroxideadded: the higher the pH within the range indicated, the larger theamount required. In practice, usually 6.5 to 15 cubic meters (S.T.P.) offlue gas having a carbon dioxide content of 6 to 15% by volume is usedper kg. of the second portion of solution. The temperature at which thecarbon dioxide treatment is carried out is not critical. It generallyranges between 20 and C.

The said second portion of the alkali metal salt solu tion, which hasbeen treated with carbon dioxide, is recycled. It is best to introduceit into the zone where alkali metal hydroxide is added to the initialsolution. It may however also be recycled to the aqueous initialsolution at a point ahead of, or advantageously after, theneutralization zone, but ahead of the point at which the said firstportion is withdrawn.

A modification of the process consists in saturating the off-gas, priorto its entering the wash tower, with water vapor at a temperature higherthan that of the gas leaving the wash tower. This means that there isthen a partial condensation of water vapor in the wash tower so that thedust particles act as condensation nuclei and can, in consequence, beparticularly easily separated. In this case, the salt solution pumped inis merely warmed owing to the condensation heat.

The process according to this invention may be carried out particularlyadvantageously if the said second portron is treated with the flue gasleaving the evaporator in which the dry alkali metal salt of thebenzenecarhoxylic acid in question is recovered from the said firstportion. In this case the alkali metal salt contained in the flue gas iswashed out by the said second portion and thus recovered. Otheradvantages are that the carbon dioxide content and the heat content ofthe flue gas are utilized.

The parts specified in the following examples are parts by weight unlessotherwise stated. The parts by weight bear the same relation to parts byvolume as the kilogram to the cubic meter.

Example I In a. continuous plant for the manufacture of terephthalicacid by thermal conversion of dipotassium phthalate in the presence ofcatalysts, an hourly yield of 220 parts of dipotassium phthalate, 10.7parts of potassium hydrogen phthalate and 2.8 parts of by-products in1460 parts of water is obtained on working up the reaction mixture. ThepH value is 5.6, and the temperature 30 C. This solution is processed ina plant such as is represented diagrammatically in the accompanyingdrawing. It is first led through feed line 1 into a vessel 2 andautomatically adjusted to pH 8.1 by means of a 50% potassium hydroxidesolution at room temperature using a pH controller fitted with a controlvalve. In this way, 5.3 parts per hour of 50 wt. percent aqueoussolution of technical grade potassium hydroxide is added.

By means of a pump 3, part of the solution (12%) is withdrawn throughline 4, while the remainder (88%) is supplied through line 6 to a washtower 7. 11 parts per hour of cadmium phthalate in the form of an about45 wt. percent aqueous suspension is added to the portion which has beenwithdrawn through line 4 and led into dryer 5. 2,000 cubic meters perhour of a flue gas obtained from the combustion of fuel oil is led intothe dryer 5. When steady-state conditions have been set up, there iswithdrawn from the dryer 245 parts per hour of solids in the form of adry salt, which contains 232 parts of dipotassium phthalate.

13,250 parts per hour of alkali metal salt solution is sprayed into thewash tower 7. The flue gas leaving the dryer 5 is passed through line 9into the wash tower 7. The gas is loaded with salt residues that havenot been separated, and enters the wash tower 7 at a temperature of 160C. The carbon dioxide content of the gas is about 12.5% by volume, itswater content about 26% by volume. Owing to heat exchange, the saltsolution sprayed in at the top is heated from 30 to 47 C., Whereas theflue gas absorbs 50 parts of water vapor per hour and leaves the top ofthe wash tower 7 through line 10 at a temperature of 47 C. In the washtower 7, 6.2 parts per hour of dipotassium phthalate is washed out, andthe salt solution absorbs 0.95 part of carbon dioxide.

The salt solution is recycled from the wash tower 7 through line 11 tothe vessel 2.

The salt'recovered in the dryer 5 may be thermally rearranged by themethod described in Example 1 of German Patent specification 1,144,254.Terephthalic acid is obtained in a yield of 94 to 97% of the theory withreference to dipotassium phthalate.

If only such an amount of caustic potash solution is added to theinitial solution that just the equivalence point of dipotassiumphthalate is reached (using a pH controller) and the solution is thenevaporated, a salt is obtained which contains varying amounts ofpotassium hydroxide. If such a salt is rearranged in the way mentionedabove, terephthalic acid is obtained in a diminished yield. For example,the yield is 87% of the theory it the initial salt contains 4 wt.percent of free alkali metal hydroxide.

Example 2 When a conversion mixture comprising 193 parts dipotassiumterephthalate which has been obtained in the manner described in GermanPatent specification No. 1,014,982 by thermally converting 231 parts ofthe dry, neutral potassium-sodium salts (of which 8% is sodium salts) ofa crude xylene oxidation mixture of isophthalic acid, terephthalic acid,phthalic acid and benzoic acid in the presence of a catalyst and carbondioxide at 445 C., is worked up to recover the terephthalic acidaccording to the procedure described in U.S. Patent 2,930,813, an amountof solution of neutral salts is obtained each hour which comprises 134.5parts isophthalate, 41.5 parts o-phthalate, 44 parts 'benzoate, 6.5parts hydrogen isophthalate, and 4.5 parts hydrogen terephthalate in1,600 parts water. The solution, which has a pH value of 5.9, is pumpedcontrinuously into vessel 2 of the same apparatus as described inExample 1.

The pH is adjusted to 8.01 by adding a 50% wt. percent aqueous alkalimetal hydroxide solution (8 wt. percent of sodium hydroxide, 92 wt.percent of potassium hydroxide). 8.25 parts of the said alkali metalhydroxide solution is required each hour. The addition of this solutionis regulated by means of a pH controller.

1,840 parts per hour of the alkali metal salt solution is withdrawnthrough line 4 and evaporated in the dryer 5 into which 2,400 cubicmeters (S.T.P.) per hour of a flue gas obtained from the combustion ofoil is introduced from below. 231 parts per hour of a mixture of alkalimetal salts, which is well suited for thermal rearrangement to alkalimetal terephthalates, is obtained.

13,160 parts per hour of the solution withdrawn from the vessel 2 issprayed into the wash tower 7. The flue gas leaving the dryer 5 ispassed into the wash tower 7 from below. This gas contains 11 parts byvolume of carbon dioxide and 25 parts by volume of water, has atemperature of C., and carries with it 6.2 parts per hour of alkalimetal salt from the dryer 5. The alkali metal salt solution enters thewash tower 7 at a temperature of 45 C. and leaves it at 53 C. All alkalimetal salt present in the gas stream is washed out in the wash tower 7.The temperature of the gas stream leaving through line 10 is 53 C.

On leaving the wash tower 7, the alkali metal salt solution, whichabsorbs 0.91 part per hour of carbon dioxide, is recycled to the vessel2 through line 11.

What we claim is:

1. A process for the recovery of alkali metal salts ofbenzene-carboxylic acids other than terephthalic acid which comprisesadding to an aqueous solution of alkali metal salts of benzenecarboxylicacids other than terephthalic acid, which solution also contains atleast one acid substance selected from the group consisting of benzoicacid and acid alkali metal phthalates and has a pH below the equivalencepoint of said alkali metal salts, such an amount of alkali metalhydroxide that the pH of said solution is 0.05 to 09 unit above theequivalence point of said alkali metal salts, and dividing the solutionobtained into two portions, one portion being evaporated for recovery ofdry alkali metal salts of said benzenecarboxylic acids and the otherportion being brought into intimate contact with a gas selected from thegroup consisting of carbon dioxide and a gas containing carbon dioxideand then being recycled to said aqueous solution of said alkali metalsalts of a benzenecarboxylic acid other than terephthalic acid at apoint ahead of that where said one portion is withdrawn, such an amountof alkali metal hydrogen carbonate being produced in said other portionby said contact with said gas that the whole excess of alkali metalhydroxide, which brings the pH of said solutions of said alkali metalsalts of benzenecarboxylic acids other than terephthalic acid to 0.05 to0.9 unit above the equivalence point, is neutralized.

2. A process as claimed in claim 1 wherein said alkali metal salt ofsaid benzenecarboxylic acid is dipotassium phthalate.

3. A process as claimed in claim 1 wherein said one portion of saidalkali metal salt solution is sprayed into hot flue gas and therebyevaporated and wherein said flue gas is used as the gas containingcarbon dioxide for treating said other portion of said alkali metal saltsolution.

References Cited by the Examiner UNITED STATES PATENTS 2,863,914 12/1958 Raecke 260-525 LORRAINE A; WEINBERGER, Primary Examiner. S. B.WILLIAMS. Assistant Examiner.

1. A PROCESS FOR THE RECOVERY OF ALKALI METAL SALTS OFBENZENE-CARBOXYLIC ACIDS OTHER THAN TEREPHTHALIC ACID WHCIH COMPRISESADDING TO AN AQUEOUS SOLUTION OF ALKALI METAL SALTS OF BENZENECARBOXYLICACIDS OTHER THAN TEREPHTHALIC ACID, WHICH SOLUTION ALSO CONTAINS ATLEAST ONE ACID SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF BENZOICACID AND ACID ALKALI METAL PHTHALATES AND HAS A PH BELOW THE EQUIVALENCEPOINT OF SAID ALKALI METAL SALTS, SUCH AN AMOUNT OF ALKALI METALHYDROXIDE THAT THE PH OF SAID SOLUTION IS 0.05 TO 0.9 UNIT ABOVE THEEQUIVALENCE POINT OF SAID ALKALI METAL SALTS, AND DIVIDING THE SOLUTIONOBTAINED INTO TWO PORTIONS, ONE PORTION BEING EVAPORATED FOR RECOVERY OFDRY ALKALI METAL SALTS OF SAID BENZENECARBOXYLIC ACIDS AND THE OTHERPORTION BEING BROUGHT INTO INTIMATE CONTACT WITH A GAS SELECTED FROM THEGROUP CONSISTING OF CARBON DIOXIDE AND A GAS COMTAINING CARBON DIOXIDEAND THEN BEING RECYCLED TO SAID AQUEOUS SOLUTION OF SAID ALKALI METALSALTS OF A BENZENECARBOXYLIC ACID OTHER THAN TEREPHTHALIC ACID AT APOINNT AHEAD OF THAT WHERE SAID ONE PORTION IS WITHDRAWN, SUCH AN AMOUNTOF ALKALI METAL HYDROGEN CARBONATE BEING PRODUCED IN SAID OTHER PORTIONBY SAID CONTACT WITH SAID GAS THAT THE WHOLE EXCESS OF ALKALI METALHYDROXIDE, WHICH BRINGS THE PH OF SAID SOLUTIONS OF SAID ALKALI METALSALTS OF BENZENECARBOXYLIC ACIDS OTHER THAN TEREPHTHALIC ACID